In the conventional fabrication of microelectronic integrated circuits (I.C.'s), a variety of dielectric films (e.g., SiO2, Si3N4), semiconductor films (e.g., epitaxial Si, polycrystalline Si, GaAs) and conductor films (e.g., W, WSi2, TiN) are deposited by chemical vapor deposition (CVD) processes. These CVD processes are well known in the semiconductor processing field and can be classified into the following categories:
Thermal CVD (CVD, LPCVD, APCVD)
In this type of process, thermal energy is used to cause a chemical reaction to occur and to cause a deposit of the desired film on a substrate. Examples of the process are as follows: ##STR1##
The temperatures required in the thermal CVD processes are generally higher than those required in the plasma enhanced CVD (PECVD) and photo CVD (PHCVD) processes described below. Also, a thermal CVD process tends to be isotropic because there is no energy in addition to thermal energy which can give direction to the chemical reaction which occurs. This contributes to the void formation in patterned geometries of small dimensions (&lt;1 um) and pitches (&lt;2 um) having large aspect ratios (&gt;1).
Some variations of the CVD processes are to carry them out at low pressures (LPCVD), e.g., 1-10 mTorr, or at atmospheric pressures (APCVD), e.g., 500-760 mTorr. The differences in the LPCVD and APCVD processes in terms of the deposition rates and film properties depend upon the reaction chemistry. However, in general, the deposition rates in a LPCVD process are lower than in an APCVD process because the density of the reactants is smaller in the LPCVD process.
Plasma Enhanced CVD (PECVD)
In this type of process, a plasma is generated to create ions, free radicals and electrons which aid the chemical reaction to occur, usually at temperatures lower than those required for thermal CVD, and to produce the desired film on the substrate. The PECVD process is done at low pressures (e.g., 1-10 mTorr) which is necessary to create and sustain the plasma. This pressure constraint is one of the disadvantages of LPCVD because the density of the reactants is less than that in APCVD, which can result in lower deposition rates in the former. Examples of PECVD process is as follows: ##STR2##
The free radicals generated in the plasma are very reactive, and their concentration is much higher than that of the ions. This can lead to gas phase nucleation of the reaction, causing unwanted particulate contamination in the film. Further, the unwanted species generated in the plasma as free radicals get incorporated in the film causing deleterious effects. The reactions occurring in a plasma process are quite complex. They depend on a variety of variables such as r.f. power, frequency, duty cycle, reactants, pressure, temperature and the design of the process chamber and electrodes of the system.
Photon-Induced CVD (PHCVD)
In this process, high-energy and high-intensity photons are used to dissociate and excite the reactant species in the gas phase for the chemical reaction to occur at rather low temperatures (e.g., even at room temperatures). The PHCVD process is done usually at near atmospheric pressure (e.g., 500-760 mTorr). For efficient transfer of the photon energy to the reactants for their excitation, catalytic agents such as mercury vapor are used for some processes. Also, lasers are used for some processes such as direct writing because of their frequency tunability and high intensity. However, PHCVD processes have not yet become production-worthy because of low density and deposition rate of, and contamination in, the deposited films.
Because of the numerous problems associated with conventional CVD as described above, a need exists to provide improvements in CVD. The present invention satisfies this need.